1. Field of the Invention
The present invention relates to methods of preparing electrocatalysts for fuel cells in a core-shell structure, and electrocatalysts.
2. Discussion of Related Art
A fuel cell is an apparatus that generates electric energy through electrochemical reaction of fuel and an oxidant. The fuel cell uses hydrogen as the fuel and oxygen as the oxidant, and an electrode includes an anode for catalyzing an oxidation reaction by hydrogen and a cathode for catalyzing a reduction reaction by oxygen. In the fuel cell, the electrode is referred to as an electrocatalyst since such catalytic actions are performed in the fuel cell. The electrocatalyst is prepared using a method of supporting particles, which serve to perform the catalytic actions, in a support such as carbon.
A catalyst material commonly used as the electrocatalyst of the fuel cell is platinum. However, platinum has a problem in that it is very expensive and should have a minimum permissible content of impurities. Therefore, much research has been conducted on preparing and using an electrocatalyst capable of reducing an amount of used platinum and giving more excellent electrochemical activity and stability than that using pure platinum. Such research has been conducted to improve the activity of platinum itself or propose an electrocatalyst formed of an alloy of platinum and a transition metal. In recent years, increasing attention has been paid to electrocatalysts that are proposed to have a core-shell structure due to particularly high electrochemical activity and stability.
However, it is difficult to prepare uniform core particles having a nanosize diameter in a process of preparing an electrocatalyst having a core-shell structure, and a key point is to homogeneously form a shell layer on a surface of each generated core particle. In particular, when the core particles are first supported in a support and shell layers are then formed, the shell layers are not only selectively formed on surfaces of the core particles, but also formed on a surface of the support. As a result, various performances may be degraded. Accordingly, an electrocatalyst having a core-shell structure may be prepared by forming nanosize core particles, coating shell particles on the core particles to prepare catalyst particles having a core-shell structure, and supporting the catalyst particles in a support. In this method, supporting the catalyst particles in the support is achieved through a physical bond between the catalyst particles and the support. Therefore, the bonding force between the support and the catalyst particles is not so strong. On the other hand, when the core particles can be directly supported in the support, a chemical bond between the support and the core particles is formed. Therefore, it is possible to support the core particles in the support with a stronger bonding force. Also, it is possible to support a much larger amount of particles.
Meanwhile, a stabilizer or a dispersing agent has been used to achieve uniformity of core particles and form a uniform shell layer during formation of a core-shell structure. Such a stabilizer affects the reactivity of a catalyst and is an obstacle to forming shell layers on surfaces of the core particles. Therefore, the stabilizer should be removed through chemical treatment or heat treatment. However, the core particles formed during such chemical treatment or heat treatment may cohere to each other, and may be deformed. Also in the case of the shell layers, the activity of the electrocatalyst may be degraded due to the cohesion of particles or the collapse of shell layers.